1. Field of the Invention
The present invention relates to an optical semiconductor device having a buried hetero-structure and to a pumping light source for optical fiber amplifier using the same.
2. Description of the Related Art
A current injection type semiconductor device adopts a structure having a current confinement function to efficiently inject current to an active layer, i.e., an emitting region. One of the current confinement structures uses a structure having buried current blocking regions on the both sides of a mesa-stripe containing the active layer. A structure including inversely biased pn junction is used in general as the current blocking region. It is noted that a difference of refractive indices is generated between the current blocking region and the active layer in the semiconductor device having such buried current blocking region, so that the semiconductor device has a horizontal optical waveguide function for confining generated light within the active layer. Due to this optical waveguide function, the shape of the generated light (laser beam) becomes substantially circular and the semiconductor device has an advantage that it may be coupled with an optical fiber with high coupling efficiency through an optical part such as a lens.
FIG. 5 is a diagrammatic view of a section of a known semiconductor device perpendicular to a direction of a resonator having a so-called buried hetero-structure (BH) having a current device blocking layer grown by metal-organic vapor phase epitaxy (MOVPE). In the semiconductor device shown in FIG. 5, a mesa-stripe is formed by an n-type InP cladding layer 2, a GaInAsP active layer 3 and a p-type InP cladding layer 4 on an n-type InP substrate 1 and the current blocking regions composed of a p-type InP current blocking layer 5 and an n-type InP current blocking layer 6 are formed on the both side surfaces of the mesa-stripe by means of buried growth. Further, a p-type InP cladding layer 7 and a p-type GaInAsP contact layer 8 are formed at predetermined position on the mesa-stripe and the current blocking regions. Then, an n-type electrode 9 is formed under the n-type InP substrate 1 and a p-type electrode 10 is formed on the contact layer 8 to inject electric current.
In the semiconductor device in FIG. 5, similarly to the pn double hetero junction including the GaInAsP active layer 3, forward bias is applied to the pn junction between the p-type InP current blocking layer 5 and the n-type InP cladding layer 2 when the semiconductor device is operative. Further, the p-type InP cladding layer 7, the n-type InP current blocking layer 6, the p-type InP current blocking layer 5 and the n-type InP substrate 1 compose a pnpn-type thyristor structure 11.
When plus voltage is applied to the p-side electrode 10 and minus voltage to the n-side electrode 9 in the semiconductor device having such structure, the most of the injected current is injected to the active layer 3 due to a difference of built-in voltage of the pn junction. However, because forward bias is applied in a path I-II in FIG. 5, a leak current is generated through the pn junction between the p-type InP current blocking layer 5 and the n-type InP cladding layer 2. This leak current turns out a gate current of the thyristor structure 11, so that a leak current flows through a path III-IV corresponding to an anode current. Heretofore, technologies aiming at reducing the leak current are disclosed in Patent Documents 1 through 5.    Patent Document 1: Japanese Patent Application Laid-open No. 2008-198662    Patent Document 2: Japanese Patent Application Laid-open No. Hei. 8-127251    Patent Document 3: Japanese Patent Application Laid-open No. Hei. 5-13869    Patent Document 4: Japanese Patent Application Laid-open No. Hei. 5-129723    Patent Document 5: Japanese Patent Application Laid-open No. Hei. 5-75209
However, the leak current generated in the current blocking region of the semiconductor device having the buried hetero (BH) structure may become a cause that induces an increase of threshold current, thermal saturation of light output and discontinuous point (kink) in the light-current output characteristics. The discontinuous point (kink) is problematic in particular because it remarkably drops high output and high temperature operations of the semiconductor device.
In view of the problems described above, the present invention aims at providing a semiconductor device whose leak current in a current blocking region is small and which is capable of performing high-output and high-temperature operations, without causing any trouble in a process for fabricating the semiconductor device.